Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL) and the neurological disorder, HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The virus preferentially targets CD4+ T cels, but also infects other secondary cell types such as CD8+ T cells, bone marrow progenitor cells, cells of the monocyte-macrophage lineage, and resident CNS cell populations. Previous studies from us and others have suggested that virus-induced alterations in these cell compartments play important roles in the genesis of the progressive neurological disorder HAM/TSP. However, little information exists concerning the molecular mechanisms of HTLV-1 promoter (long terminal repeat or LTR) regulation in these cells. Moreover, nearly all studies highlighting the importance of the cellular transcription factors in HTLV-1 Tax-mediated LTR activation and the ability of Tax protein to interact with these factors independently have been performed using transiently transfected viral reporter plasmids or in cell lines that do not represent the primary target for HTLV-1 in vivo. Studies performed with HIV-1 have indicated that the integrated provirus differs from a transfected viral plasmid both physically and in the requirement for certain cellular factors, especially those belonging to the chromatin remodeling histone acetyltransferase (HAT) family. Hence, to better understand HTLV-1 gene regulation and the complex interplay with the integrated provirus, we previously characterized a number of stable clones of CD4+ T-cell (Jurkat), monocyte (U-937), and progenitor (TF-1) cell lines carrying integrated copies of the HTLV-1 LTR driving luciferase gene expression. Preliminary investigations using these clones have highlighted novel interactions between the Tax protein and host microRNAs involved in regulating chromatin remodeling leading to the proposed studies. Our results are consistent with a novel hypothesis that Tax can modulate the cellular miRNA machinery in a cell-type specific manner involving chromatin remodeling effecting viral gene expression controled by the HTLV-1 LTR. The Specific Aims to validate this concept and to test our hypothesis are to (1) Investigate the mechanism of Tax- mediated DNA-protein interactions in the context of chromatin in primary and secondary target cell populations during the course of viral disease, (2) Define HTLV-1 Tax-modulated host miRNA expression linked to chromatin remodeling in primary and secondary cell populations targeted by HTLV-1, and (3) Validate role of Tax-miRNA-chromatin interactions in HTLV-1 disease in patient cohort(s). The proposed studies ofer the potential to answer basic key questions related to HTLV-1 pathogenesis and will provide novel insight into why many CD4+ T cells in infected individuals containing HTLV-1 proviral sequences do not express viral proteins. In addition, these studies will provide a comparative account of molecular mechanism(s) related to HTLV-1 gene regulation in T cells versus bone marrow progenitor cells and cells of the monocyte-macrophage lineage, which are critical cellular components involved in retroviral neuropathogenesis.